Analysis of the Combustion Characteristics of a HCCI Engine Operating on DME and Methane

The Homogeneous Charge Compression Ignition (HCCI) engine has attracted much interest in recent years because it can simultaneously achieve high efficiency and low emissions. However, it is difficult to control the ignition timing with this type of engine because it has no physical ignition mechanism. Varying the amount of fuel supplied changes the operating load and the ignition timing also changes simultaneously. The HCCI combustion process also has the problem that combustion proceeds too rapidly. This study examined the possibility of separating ignition timing control and load control using an HCCI engine that was operated on blended test fuels of dimethyl ether (DME) and methane, which have vastly different ignition characteristics. The influence of the mixing ratios of these two test fuels on the rapidity of combustion was also investigated. Moreover, as a basic research subject, the behavior of formaldehyde (HCHO), an intermediate that is produced by cool flame reactions, was observed by spectroscopic techniques. The experimental results revealed that, within the range of the experimental conditions used in this study, the quantity of DME supplied substantially influenced the ignition timing, whereas it was little affected by the quantity of methane supplied. This indicates that the ignition timing can be controlled by the quantity of DME supplied and that the load can be adjusted by the quantity of methane supplied. Spectroscopic measurements of the behavior of a substance corresponding to HCHO also indicated that the quantity of DME supplied significantly influenced the cool flame. However, the rapidity of combustion could not be controlled even by varying the mixing ratios of DME and methane. It was made clear that changes in the ignition timing substantially influence the rapidity of combustion.